difficulty in maintaining water quality

Question
hi,

i have very difficult time maintaining the quality of the water in my 8.5 gallon aquarium.

i bought koi carps a month ago, my aquarium had only air stone and no gravels or filter. later within a week i replace the air stone with a filter containing fibre glass as filter media. i added four guppies in the aquarium, bt one of them died after 2 weeks, later all other guppies died in a course of a week.

during ths week i had fed thm blood worms and now i fear tht thay were infected with some parasite. befor each guupy died, it had swollen belly and some kind of white tread near their anus. do u think that the parasite are still there in the water.

i do partial water change every 4 days. shud i perform tear down of the aquarium to get rid of parasites or other infectious agents. fhpls let me know the procedure for the same.

thanking you
regards

Answer
Shraddha,

The first problem is the tank size. It is way too small for Koi. Koi are fish that get very big and really should be in a pond. Koi are cold water fish where guppies are tropical and need a heater. I doubt that the blood worms has the parasites, but either the guppies or the Koi could have had them before you bought them. You never want to treat a fish with any medication unless you are 100% sure what the problem is. If the guppies were swollen, they could have suffered from swim bladder disease which is caused by over feeding. I would do the following.

1. get a larger tank for the Koi.
2. make sure the filter is large enough for the tank.
3. Do not mix cold water fish with tropical fish.
4. Inspect the tank closely to see if you spot any parasites.

If you have any other questions, please don't hesitate to ask.